How to define @OneToMany in parent entity when child has composite PK?

Question

My Parent class has two child classes: Child and ParentHobby. The Child class has a singular PK and the @OneToMany mapp

Answer 1

You need to use a derived identity.

ParentHobbyPK should look like this:

@Embeddable
public class ParentHobbyPK {
    @Column(name="HOBBY_ID")
    private String hobbyID;
    private long parentID; // corresponds to the PK type of Parent
}

ParentHobby should look like this (the important thing being the @MapsId annotation):

@Entity
@Table(name="PARENT_HOBBY")
public class ParentHobby {
    @EmbeddedId
    private ParentHobbyPK id;

    @MapsId("parentID") // maps parentID attribute of the embedded ID
    @ManyToOne(optional = true)
    @JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID", insertable = true, updatable = true)
    private Parent parent;

    ...
}

Derived identity is discussed in JPA 2.1 spec, section 2.4.1.